The present invention relates to a steel bar or wire rod, for cold forging, used for manufacturing machine structural components such as those of cars and construction machines and a method to produce the same. More specifically, the present invention relates to a steel bar or wire rod, for cold forging, excellent in ductility and suitable for the cold forging by heavy working and a method to produce the same.
Carbon steels for machine structural use and low alloy steels for machine structural use have conventionally been used as structural steel materials for manufacturing machine structural components such as those of cars and construction machines. Machine structural components such as bolts, rods, engine parts and driving system components for cars have so far been manufactured from these steel materials mainly through hot forging and machining processes. A recent trend, however, is that the above processes are replaced with a cold forging process for the sake of enhanced productivity and other advantages. In a cold forging process, cold forging is usually applied to hot rolled steel materials after spheroidizing annealing (SA) is applied to secure cold workability. A problem in the cold forging is, however, that the steel materials are hardened by working and their ductility is lowered, resulting in the occurrence of cracks and a shorter service life of metal dies. In case of heavy cold forging in particular, cracking during cold forging, namely the insufficient ductility of steel materials, is often the main hindrance to changing the process from hot forging to cold forging.
Meanwhile, since the spheroidizing annealing (SA) requires high temperature heating and a long retention time of steel materials, it not only requires a heat treatment facility such as a reheating furnace but also consumes energy for the heating, and therefore the process accounts for a large proportion of the total manufacturing cost. To cope with this, various technologies have been proposed from the viewpoints of productivity improvement, energy saving, etc.
Some examples are as follows: Japanese Unexamined Patent Publication No. S57-63638 proposes a method to shorten the time for spheroidizing annealing and obtain a steel wire rod excellent in cold forging by cooling a steel material to 600xc2x0 C. at a cooling rate of 4xc2x0 C./sec. or higher after hot-rolling to form a quenched structure and then applying spheroidizing annealing to the steel material covered with scale in an inert gas atmosphere; Japanese Unexamined Patent Publication No. S60-152627 proposes a method to enable quick spheroidizing by regulating finish rolling conditions, rapidly cooling the steel material after the rolling and forming a structure in which fine pearlite, bainite or martensite is intermingled with finely dispersed pro-eutectoid ferrite; Japanese Unexamined Patent Publication No. S61-264158 proposes a method to lower the steel hardness after spheroidizing annealing by improving steel chemical composition, namely obtaining a low carbon steel having a reduced P content of 0.005% or less and satisfying Mn/Sxe2x89xa71.7 and Al/Nxe2x89xa74.0; and Japanese Unexamined Patent Publication No. S60-114517 proposes a method to eliminate a softening annealing process before cold working by applying a controlled rolling.
All these conventional technologies aim at improving or eliminating the spheroidizing annealing before cold forging and do not aim at improving the insufficient ductility of steel materials, which is the main hindrance to changing the process from hot forging to cold forging in the manufacture of machine components requiring heavy working.
In view of the above situation, the object of the present invention is to provide a steel bar or wire rod for cold forging excellent in ductility after spheroidizing annealing and capable of preventing the occurrence of cracking in the steel material during cold forging which has, so far, been a problem when manufacturing machine structural components by cold forging after applying spheroidizing annealing to a hot-rolled steel bar or wire rod, and a method to produce the same.
The inventors of the present invention discovered, as a result of investigating the cold workability of a steel bar or wire rod for cold forging, that it was possible to obtain a steel bar or wire rod for cold forging excellent in ductility after spheroidizing annealing by hardening only the surface layer of a steel bar or wire rod having a specific chemical composition and softening the structure of its center portion.
The gist of the present invention, which has been established on the basis of the above finding, is as follows:
(1) A steel bar or wire rod for cold forging excellent in ductility after spheroidizing annealing, having a chemical composition comprising, by mass,
0.1 to 6.65% of C,
0.01 to 0.5% of Si,
0.2 to 1.7% of Mn,
0.001 to 0.15% of S,
0.015 to 0.1% of Al,
0.0005 to 0.007% of B, and
the restricted elements of
0.035% or less of P,
0.01% or less of N and
0.003% or less of O,
with the balance consisting of Fe and unavoidable impurities, characterized in that: the area percentage of ferrite structure is 10% or less at the portion from the surface to a depth of 0.15 times the radius of the steel bar or wire rod; the other portion consists substantially of one or more of martensite, bainite and pearlite; and further the average hardness of the portion from the depth of 0.5 times its radius to its center is less than the hardness of its surface layer (the portion from the surface to the depth of 0.15 time the radius) by HV 20 or more.
(2) A steel bar or wire rod for cold forging excellent in ductility after spheroidizing annealing according to the item (1), characterized by further containing 0.2 mass % or less of Ti.
(3) A steel bar or wire rod for cold forging excellent in ductility after spheroidizing annealing according to the item (1) or (2), characterized by further containing, by mass, one or more of
3.5% or less of Ni,
2% or less of Cr and
1% or less of Mo.
(4) A steel bar or wire rod for cold forging excellent in ductility after spheroidizing annealing according to any one of the items (1) to (3), characterized by further containing, by mass, one or both of
0.005 to 0.1% of Nb and
0.03 to 0.3% of V.
(5) A steel bar or wire rod for cold forging excellent in ductility after spheroidizing annealing according to any one of the items (1) to (4), characterized by further containing, by mass, one or more of
0.02% or less of Te,
0.02% or less of Ca,
0.01% or less of Zr,
035% or less of Mg,
0.1% or less of Y and
0.15% or less of rare earth elements.
(6) A steel bar or wire rod for cold forging excellent in ductility after spheroidizing annealing 15 according to any one of the items (1) to (4), characterized in that.the austenite grain size number according to Japanese Industrial Standard (JIS) is 8 or larger at the portion from the surface to the depth of 0.15 times the radius of the steel bar or wire rod.
(7) A method to produce a steel bar or wire rod for cold forging excellent in ductility after spheroidizing annealing, characterized by finish-hot-rolling a steel having a chemical composition specified in any one of items (1) to (5) in a manner to control its surface temperature to 700 to 1,000xc2x0 C. at the exit from the final finish rolling stand and then subjecting it to at least one or more process cycles consisting of rapid cooling to a surface temperature of 600xc2x0 C. or below and recuperation by its sensible heat to a surface temperature of 200 to 700xc2x0 C., so that the area percentage of ferrite structure is 10% or less at the portion from the surface to the depth of 0.15 times the radius of the steel bar or wire rod, the other portion consists substantially of one or more of martensite, bainite and pearlite, and further the average hardness of the portion from the depth of 0.5 times its radius to its center is less than the hardness of its surface layer (the portion from the surface to the depth of 0.15 times the radius) by HV 20 or more.
(8) A steel bar or wire rod for cold forging excellent in ductility, characterized in that the steel bar or wire rod is subjected to spheroidizing annealing like any one of the items (1) to (6), the degree of spheroidized structure defined by JIS G 3539 is within No.2 at the portion from the surface to the depth of 0.1.5 time the radius of the steel bar or wire rod and, in addition, the degree of spheroidized structure is within No. 3 at the portion from the depth of 0.5 time its radius to its center.
(9) A steel bar or wire rod for cold forging excellent in ductility according to the item (8), characterized in that the ferrite grain size number under JIS is 8 or larger at the portion from the surface to the depth of 0.15 times the radius of the steel bar or wire rod.